docs: def popone(self, key, default=_marker):
          """Remove specified key and return the corresponding value.
  
          If key is not found, d is returned if given, otherwise
          KeyError is raised.
  
          """
          identity = self._title(key)
          for i in range(len(self._impl._items)):
              if self._impl._items[i][0] == identity:
                  value = self._impl._items[i][2]
                  del self._impl._items[i]
                  self._impl.incr_version()
                  return value
          if default is _marker:
              raise KeyError(key)
          else:
              return default